The present study will examine the use of the ratio of 25-O-desacetylrifabutin to rifabutin as a measure of cytochrome P450 3A4 activity. This will be accomplished by timed plasma collection after rifabutin administration before and after administration of CYP3A4 inhibitor, clarithromycin. The change in the ratio of 25-O-desacetylrifabutin to rifabutin secondary to CYP3A4 inhibitor will be correlated to the change in the 14C erythromycin breath test.